Here is the full list of all 20 Atomik: RunGunJumpGun achievements.
No enemy ships destroyed in 'Target Locked'
Beat a level without using the fly button.
No mercy for turrets in 'Pew Pew Pew'
No saws dislodged in 'Stomach II'
Beat every level in World 1
Die 100 times on one level
Beat every level in World 2
Beat every level in World 3
Collect all Atomiks in World 1
Collect All Atomiks in World 2
Collect All Atomiks in World 3
Beat Omegalith
Beat Omegastroid
Beat Omegaton
Collect all Atomiks
Complete a World 1 Marathon Run
Complete a World 2 Marathon Run
Complete a World 3 Marathon Run
No enemy ships harmed in 'Gridiron'
Shoot the saw through the two large chambers and live to tell about it in 'Football Racetrack'